It's just the z co-ordinate was not in any way included in the gaussian matrix, therefore I can't see any way that the z co-ordinates played a part in determining whether the 3D lines intersect. I can extract the z value at the end just fine, but the [0,1] lambda value (and the other one) was only determined from the x and y co-ordinates. Do you know what I mean? Sorry if I am not explaining this well. I trust you know better than I do, please correct me if I'm wrong.